One of the therapeutic procedures applicable to the present invention is known as percutaneous transluminal coronary angioplasty (PTCA). This procedure can be used, for example, to reduce aerial build-up of cholesterol fats or atherosclerotic plaque. Typically a guidewire is steered through vascular system to the site of therapy. A guiding catheter, for example, can then be advanced over the guidewire and a balloon catheter advanced within the guiding catheter over the guidewire. The balloon at the distal end of the catheter is inflated causing the site of the stenosis to widen. The original catheter can then be withdrawn and a catheter of a different size or another device such as an atherectomy device can be inserted.
The major considerations in guidewire design include steerability, flexibility, medial stiffness or support, bending in transition areas, tip formability and radiopacity. In a typical guidewire construction a tapered stainless steel core wire has a platinum spring coil wound around the tapered distal end of the core wire. The tapered area of the core wire is called the transition segment. The longer the tapered transition segment, the more flexible the guidewire. A blunt tip is typically welded to the distal end of the guidewire to reduce trauma to the blood vessel.
Support refers to a guidewire's ability to provide a strong "platform" or track for the catheter to move over as it is crossing the lesion. Support becomes crucial when the lesion is tight. Catheters are soft and rely heavily on the support provided by the guidewire. The spring coil is typically used to provide device support and maintain a consistent guidewire outer diameter. If the outer diameter of the guidewire is reduced, it will exert more force per unit area and may result in cutting through the blood vessel rather than tracking through the bends in the vessels. Increasing core wire diameter also assists in providing enhanced support. The spring coil wire is wound into a coil and placed over the core wire. The spring coil proximal end is difficult to attach to the core wire. A typical spring coil is approximately 0.002 inches in diameter thereby providing a very small area with which to attach to the core wire. Spring coil guidewire construction has been known in the art for many years. An early example of such guidewire construction includes U.S. Pat. No. 3,789,841 for a "Disposable Guide Wire" to Antoshkiw.
Transition refers to areas of changing diameter along the guidewire. A smooth transition gives the guidewire the ability to follow itself smoothly around vascular bends. If a stiffer portion of the guidewire behind the flexible tip does not follow the tip around vascular bends, the tip position may be lost. A guidewire with poor or rough transition will show elbows or bends in the vascular curves. Without smooth transitions a guidewire will not corner smoothly. Smooth transitions also facilitate the tracking of the balloon catheter over the wire when crossing the lesion.
U.S. Pat. No. 4,884,5791 to Engelson for "Catheter Guidewire" discloses a guidewire with proximal, intermediate and distal sections. The intermediate section has greater lubricity than the adjacent proximal and distal sections. The greater frictional coefficient in the distal end segment acts to anchor the end of the wire in a branch vessel when the guide wire has been advanced across the sharp-bend vessel junction. In FIG. 6, the distal segment of the core wire is incased in a polymer tube having a series of annular grooves to provide increased tube flexibility as well as greater frictional coefficient.
Elastomers and shape memory materials have been used in the catheter industry to promote elasticity and to promote tips that will return to a preformed curve after flexing. Super-elastic guidewires are known in the art, as for example, U.S. Pat. No. 4,925,445, to Sakamoto et al. for "Guide Wire for Catheter" which discloses a guidewire with at least portions of the inner core formed of the super-elastic metallic member. U.S. Pat. No. 5,067,489 to Lind for "Flexible Guide with Safety Tip" discloses an elongated, helically wound coil and an elongated flexible metal core of shape memory alloy. U.S. Pat. No. 5,069,226 to Yamauchi et al. for "Catheter Guidewire with Pseudo Elastic Shape Memory Alloy" discloses a catheter guide wire comprising a solid core wire of Ti-Ni shape memory alloy and an outer jacket coveting the core wire. The jacket is made of any one of synthetic resins such as polyethylene, polyvinylchloride, polyester, polypropylene, polyamide, polyurethane, polystyrene, fluoride resin, silicone rubber, and other elastomers. U.S. Pat. No. 5,243,996 to Hall for "Small-Diameter Superelastic Wire Guide" discloses a mandrel of metallic superelastic material, such as nitinol having a smoothly rounded tip attached to the distal tip of the mandrel and a coil attached at the distal region of the mandrel, the coil coaxially surrounding a portion of the distal region.